Argon plasma coagulation (APC) or argon beam coagulation (ABC) is a known surgical technique for controlling surface bleeding in a manner that does not require physical contact between a surgical probe delivering the plasma and the lesion. APC can be performed endoscopically, whereby a jet of argon gas is directed through a probe passed through an endoscope. Ionization of the argon gas as it is emitted creates the plasma that causes coagulation.
To strike plasma it is desirable to have a high electric field (e.g. by directly applying a high voltage or setting up a high impedance condition that causes an high voltage to exist). Typically this is done by applying a high RF voltage pulse (e.g. 500 V to 2 kV) between an active electrode and a return electrode that are separated by a small distance, e.g. less than 1 mm, for a short duration of time, e.g. in a range from 1 ms to 10 ms. The high electric field can break down the gas to initiate a plasma. In one embodiment discussed in WO 2009/060213, a high voltage (high impedance) condition is set up using a flyback circuit that uses a low frequency (e.g. radiofrequency) oscillator circuit e.g. running at 100 kHz and a transformer whose primary winding is connected to the low voltage oscillator circuit by a suitable driver and switching device (e.g. gate drive chip and a power MOSFET or BJT). The arrangement generates high voltage pulses or spikes which strike or otherwise initiate the plasma. Once struck, the impedance drops and the plasma may be maintained by a supply of microwave energy.
Polyps in the GI tract can be removed using a medical snare in an endoscopic procedure, e.g. using a colonoscope. In the case of pedunculated polyps, the snare is passed over the polyp and tightened around the polyp's neck to remove the polyp. The polyp neck may be cut by passing a radiofrequency (RF) current through the biological tissue. The current may also facilitate cauterisation.
Sessile polyps can be removed in a similar manner. It is preferable to “plump up” such polyps before removal by injecting saline or sodium hyaluronate, under the polyp to raise it away from the surrounding colon wall. This may help to reduce the risk of bowel perforation.
It is known to incorporate electrodes into the loop of a snare in order to provide an integrated means for delivering the RF current. Both monopolar, for use with a separate ground pad attached to the patient, and bipolar arrangements are known.
A disadvantage of known RF cutting snares is the high level of electrical power (in particular the use of high voltages) needed to initiate cutting action, as it carries with it the risk of unwanted thermal damage to the bowel wall. For example, the peak voltage associated with monopolar and bipolar coagulation may be in excess of 4,500 V and 450 V respectively. Moreover, the use of monopolar devices gives little or no control over the current path, which means energy may be not be accurately or controllably delivered to the desired regions of tissue.